This invention relates to adducts prepared by the thermal addition of maleic anhydride to elastomeric coplymers of ethylene and propylene which have a substantially saturated hydrocarbon backbone chain and unsaturated hydrocarbon side-chains. This invention also relates to thermoplastic elastomers prepared from such adducts.
Grafting of maleic anhydride to polyolefins by free radical initiated reactions is well known in the art. U.S. Pat. No. 3,236,917 to Natta et al, for example, discloses adducts prepared by heating a mixture of ethylene/propylene copolymer and maleic anhydride in the presence of an organic peroxide which initiates the addition reaction by the generation of free radicals. Among other reactions, a molecule of the maleic anhydride grafts onto two copolymer chains thereby cross linking the polymer. This cross-linking is irreversible. The adduct may be further cross-linked by a basic compound such as zinc oxide.
Numerous other suggestions have been made for grafting maleic anhydride to synthetic elastomers, followed by cross-linking with a metal oxide to form a thermoplastic elastomer. U.S. Pat. No. 3,644,248 to Luijk, for example, discloses addition of maleic anhydride to polyisoprene during mastication or in the presence of an organic peroxide followed by cross-linking with a Group II or IV divalent metal oxide.
Such processes utilize free radicals, either generated by shearing stresses during mastication or by heating organic compounds such as peroxides, to prepare the maleic anhydride/elastomer adduct. The free radical mechanism of adduct formation, however, has certain disadvantages. Namely, free radicals cause a molecule of maleic anhydride to add to two elastomer molecules, thereby cross-linking the elastomer. Free radicals can also react with the elastomer to introduce cross-linking. The degree of cross-linking in raw elastmeric polymers is commonly expressed in terms of gel content.
Uncontrolled cross-linking, as shown by a high gel content, tends to cause the uncured elastomeric polymer to have poor processing characteritics such as poor milling properties and slow extrusion rates. Poor tensile strengths and short break are also commonly observed in cured elastomers derived from elastomeric polymers which, in the raw or uncured state, have high gel content.
The art has proposed use of free radical inhibitors to control cross-linking during the additon of maleic anhydride to elastomers having polyunsaturated backbones. Thus, Canadian Pat. No. 543,006 discloses the addition of maleic anhydride to Hevea rubber in the presence of a free radical inhibitor. Resulting adduct, when prepared in a press or autoclave, is still preferably treated with breakdown aids to reduce cross-linking prior to subsequent compounding and processing.